In the use of nuclear reactors, such as pressurized water nuclear reactors, measurements must be performed in the core consisting of the fuel assemblies, while the reactor is in operation. In particular, measurements of neutron flux must be performed in various places along the height of the core, so as to ascertain the neutron flux or power distribution along the axial direction of the core, which is generally the vertical direction.
The fuel assemblies of which the core consists comprise, in their central part, an instrumentation tube into which there may be introduced, over the entire height of the core, a leakproof guide conduit, inside which a neutron flux measurement probe, attached to a flexible cable, is moved while the reactor is in operation. The guide conduits associated with each of the fuel assemblies in which flux measurements are carried out must be capable of being withdrawn from these assemblies, for example when the core is being refuelled. These guide conduits are therefore mounted so that they can move by sliding inside the guide tubes connecting the lower part of the vessel to a measuring room, in which the ends of the guide conduits remote from the ends introduced into the core are accessible for moving the probes, for collecting measurement signals and for moving the guides conduits inside the guide tubes The movement of the guides conduits in the guide tubes is performed merely by pushing or pulling, sufficient clearance being provided to limit the forces to be applied to the guide conduit, despite the curved shape of the guide tubes over a large proportion of their trajectory
In a known and widely employed embodiment, the trajectory of the guide conduit is in the shape of a U from the upper part of the guide tube of the corresponding fuel assembly as far as its end entering the measuring room.
The tube for guiding the guide conduit comprises successively a vertical first part attached to the bottom of the vessel, in the extension of a passage channel passing through a part of the lower internal equipment of the reactor and of the guide tube of the corresponding fuel assembly, a horizontal second part and a vertical third part whose upper end is attached to the structure of the nuclear reactor and carries a passage device permitting the leakproof passage of the guide conduit whose upper end part corresponding to the upper part of the second leg of the U, vertical in direction, is situated outside the guide tube. This upper end part of the guide conduit emerges inside the measuring room and has an open end through which a measurement probe attached to the end of a flexible cable can be introduced into the guide conduit.
The leakproof passage device attached to the upper part of the guide tube is positioned slightly above the height of the reactor vessel, so that, during reactor maintenance, when the vessel and the reactor pool are filled with water, this water cannot reach the end of the guide tube in the case where the guide conduit is completely withdrawn from the guide tube. Leakages into the room receiving the end parts of the guide tubes are thus avoided.
When the reactor is in operation, the guide tube is filled with primary fluid at high temperature and under pressure, so that the pressure of this fluid, of the order of 155 bars, is applied to the outer surface of the guide conduit introduced into the guide tube. The leakproof passage device of the guide conduit makes it possible to avoid any leakage of primary water under pressure at the upper part of the guide tube, the sealing being ensured by seals of the passage device in leakproof contact with the outer surface of the guide conduit.
A passage device permitting the leakproof passage of guide conduits of a pressurized water nuclear reactor is describe in applicant's patent FR-A-2,080,077.
The wall of the guide conduit is normally completely leakproof, but can be damaged during use and, in particular, when the guide conduit is being inserted into, or withdrawn from, the guide tube.
In addition, the outer surface of this wall is exposed to a fluid at a high temperature and high pressure, circulating at a high speed in the reactor; this can cause some wall deterioration of the guide conduits, due to a mechanical or thermal phenomenon, or by corrosion. This may result in guide conduits sealing defects, giving rise to a leakage of the primary fluid inside the guide conduit.
Some primary fluid is thus liable to reach the vertical upper part of the guide conduit, which is situated above the leakproof passage device. A manual isolation valve is therefore arranged at the open upper end of each of the guide conduits, allowing the guide conduit to be shut off when it is no longer leakproof.
The closing of this valve requires the intervention by an operator inside the room where the leakproof passage device and the vertical upper end part of the guide conduit are located. As soon as the leakage appears in the guide conduit, contaminated primary liquid spreads into the room, and this enables a leakage in the guide conduit to be detected, requiring the intervention by the operator. The presence of contaminated liquid in the room where the operator intervenes makes this intervention relatively hazardous and can entail some irradiation of the personnel responsible for the surveillance and maintenance of the nuclear power station.
This intervention is all the more hazardous since the outer wall of the damaged guide conduit generally remains subjected to the pressure of the primary fluid in the reactor in operation.
There are known devices for closing a conduit for guiding a measurement probe, which are employed in a pressurized water nuclear reactor, and which are constructed in the form of a ball valve.
There is known, for example, a device of this type, comprising electromagnetic actuating means enabling the ball to be held in a closing position when the measurement probe is not introduced into the conduit.
A device of this kind, which requires actuating means of an active type, offers insufficient safety in operation, insofar as the actuating means may become accidentally unavailable.
There is also known a device consisting of a ball valve in which the ball automatically drops to a closing position by means of gravity when the measurement probe is not introduced into the guide conduit. However, a passively operating device of this kind must be placed in a vertical part of the tube for guiding the guide conduit, which is situated in the vicinity of the vessel bottom. This device consequently does not ensure the closure of the upper end of the guide conduit in the vicinity of the measuring room.